1. Field of the Invention
The invention concerns pressure-sensitive adhesive tape, especially one having a pressure-sensitive adhesive layer which is foamlike by virtue of microbubbles as taught in U.S. Pat. No. 4,223,067 (Levens).
2. Description of the Related Art
Foam-backed pressure-sensitive adhesive tape is commonly used to adhere an article to a substrate. The foam backings of such tapes often are pigmented with carbon black to afford a dark appearance that camouflages their presence.
The pressure-sensitive adhesive tape of the above-cited Levens patent has a foamlike appearance and character, even though it is not a foam, and is useful for porposes previously requiring a foam backed pressure-sensitive adhesive tape. A foamlike tape of the Levens patent now on the market is made by ultraviolet polymerization of a layer of an adhesive-forming mixture containing colorless glass microbubbles which act as a white pigment that makes the tape undesirably visible in uses such as sealing a skylight or attaching body-side moldings to automotive vehicles or simulated mullion bars to glass windows. Because of its superior performance characteristics, the foamlike tape of the Levens patent is often preferred to foam-backed tapes for such purposes and would be more acceptable if its adhesive layer were sufficiently dark to camouflage the tape. If carbon black or other pigment were added to the photopolymerizable adhesive-forming mixture in amounts sufficient to produce a desirably dark appearance, this would block the ultraviolet radiation from polymerizing the mixture to a pressure-sensitive adhesive state. Up to about 0.06 percent by weight of carbon black can be employed without undue interference with the polymerization of a 1.0-mm layer (as evidenced by adhesive and cohesive values), but such small amounts produce only a pastel grey color that would be undesirably noticeable for uses such as those mentioned above. When other pigments are substituted for the carbon black, they also cannot be used in amounts producing a color deeper than pastel.
Although the microbubbles of the Levens patent examples are glass, they can be polymeric of the type described in U.S. Pat. No. 3,651,972 (Morehouse et al.) or U.S. Pat. No. 4,287,308 (Nakayama et al.) Because these polymeric microbubbles are smaller in size and less dense than are commercially available glass microbubbles, a greater number of the polymeric microbubbles is required to attain the same volume loading. Hence, pigmented Levens-type tapes tend to be even lighter colored when filled with polymeric microbubbles than when filled with glass microbubbles.
Also useful for purposes previously requiring a foam-backed pressure-sensitive adhesive tape is the celluar pressure-sensitive adhesive tape disclosed in U.S. Pat. No. 4,415,615 (Esmay et al.). It preferably is made by polymerizing a coated froth in situ by exposure to ultraviolet radiation. Although the use of pigments is suggested (col. 5, lines 48-57), the ultraviolet radiation would be blocked if the froth contained appreciable amounts of carbon black or other pigment.
3. Other Prior Art
Since at least as early as 1941, finely divided silica has been used for the thickening of liquids, usually in a hydrophilic form, but also in hydrophobic form. See, for example, "Technical Bulletin Pigments" No. 6 of Degussa Corp., Teterboro, N.J., dated March 1981, the cover of which shows the schematic structure of a hydrophobic silica. Another Degussa bulletin, "Precipitated Silicas and Silicates" dated Dec. 1978, lists a number of hydrophilic and hydrophobic silicas and gives uses for specific silica products. A 4-page Degussa bulletin entitled "Product Information" marked "PL/sk 6/4/84", indicating the date June 4, 1984, reports that the hydrophobic silica "Aerosil" 972 is made from "Aerosil" 130, a hydrophilic silica which has a surface area of 130 m.sup.2 /g; the hydrophobic silica "Aerosil" R974 is made from "Aerosil" 200, a hydrophilic silica which has a surface area of 200 m.sup.2 /g; and the hydrophobic silica "Aerosil" R976 is made from "Aerosil" 300, a hydrophilic silica which has a surface area of 300 m.sup.2 /g. Because the finer silicas are more difficult to treat, more free hydroxyls remain on the surfaces of the finer silicas, namely 30%, 40%, and 50% on the hydrophobic R972, R974, R976, respectively.
U.S. Pat. No. 3,024,146 (Bueche et al.) concerns a certain silicone rubber composition which contains a hydrophobic silica, specifically a trialkylhalogenosilane-treated silica aerogel, and provides adhesive bonds that better resist repeated freezing and thawing in the presence of water. If one instead employs a silica which has been treated to become hydrophobic by forming silicon-bonded alkoxy groups on the surface of the silica particles, "one will not obtain the same properties or results" (col. 3, lines 26-33). While the adhesives of the examples are pastes, they also can be used as "pressure-sensitive adhesive tapes" (col. 6, lines 10-15, and col. 5, lines 59-70).
No other mention has been found in the prior art concerning the use of hydrophobic silica in pressure-sensitive adhesive tape. The above-cited Levens and Esmay patents employ "fumed silica" in the pressure-sensitive adhesives of their tapes, but "fumed silica" as produced is hydrophilic.
U.S. Pat. No. 4,163,091 (Schulz) teaches that fume(d) silica is a reinforcing filler in certain self-adhesive silicone elastomers, and that the silica "can IO be treated with organosilicon materials such as chlorosilanes, silazanes, alkoxysilanes and cyclic siloxanes to produce hydrophobic surfaces" (col. 6, line 7-52).